GB2122458A - Stereophonic television receivers - Google Patents

Abstract

In a stereophonic television receiver in which a pilot signal discriminator 6 detects a pilot signal and controls a switching circuit SW0 to discriminate the sound broadcast content (stereo, bilingual or monaural) in accordance with the modulation of the pilot signal, a noise detection circuit 11 including a filter 7,8 extracts a noise component and controls the pilot signal discriminator 6 so as to keep switch SW0 in a predetermined state when the output from detector 9 has a level higher than a predetermined level. As described the pilot signal discriminator is held in the monaural mode when the noise exceeds the predetermined level to prevent false actuation by noise signals generated, eg when the second stereo subchannel signal is absent. The filter 7,8 may respond to a beat frequency between the two sound carriers. <IMAGE>

Description

SPECIFICATION Stereophonic television receivers This invention relates to stereophonic television receivers.

Sound multiplex systemsfortelevision broadcasting have been developed in various countries. For example, a sound multiplex system used in West Germany has frequencies arranged as indicated diagrammatically in Figure 1 ofthe accompanying drawings, to which reference is now made.

A conventional sound carrier wave which has a frequency higherthan that of a video carrier wave P by 5.5 MHz is used as a first sound carrier wave S1 for a main channel. A second sound carrier wave S2 which has a frequency higher than that of the video carrierwave P by 5.742 MHz is superimposed on the sound carrier wave S1 so asto perform sound multiplexing. The first and second sound carrier waves S1 and S2 are used forstereo broadcasting, bilingual broadcasting, and monaural broadcasting in a manner as shown in thefollowing table, using frequency modulation.In order to discriminate whetherthetransmitted mode is the stereo, the bilingual or monaural mode, an amplitude modulated carrier wave is superimposed on the second sound carrier wave S2. The pilot carrier wave is amplitude modulated by a low-frequency discrimination signal (117 Hzforstereo broadcasting, 274 he for bilingual broadcasting, and 0 Hz for monaural broadcasting).

TABLE Main channel Sub-channel Stereo broadcasting (R+L)/2 R Bilingual broadcasting A B Monaural broadcasting A A R: right-channel sound L: left-channel sound A: mother language B: foreign language A stereophonic television receiver for receiving the first and second sound ca rrier waves S1 and S2 selects the output states of the main channel and the sub-channel audio signals.

A pilotsignal discriminator extracts the pilot signal included in the frequency demodulated signal from the second sound intermediate frequency amplifier.

In other words, the pilot signal discriminator detects the discrimination pilot signal which amplitude modulates the pilot carrier wave, so as to discriminate the broadcast content as being stereo, bilingual or monaural, respectivley. Aswitching circuit is then controlled in accordance with the resultofthe discrimination. Therefore, main and sub audio channel signalsfromthe first and second sound intermediate frequency amplifiers, and a stereo sound signal obtained from these signals, are selectively produced.

However, with this system, even if only the first sou nd,carrier wave S1 is present, the second sound intermediatefrequency amplifier and the discriminatorare kept in the stand-by mode.

When a monaural sound reproduction apparatus is connected to the system, various types of noise, such as spurious noise are radiated. Such noise sometimes corresponds to the frequency of the pilot signal.

In particular, when a video tape recorder is connected tothesystem and it generates higher harmonic noise due to a noise bar in the "cue and review" mode, the pilot signal discriminatorofthetelevision receiver may be erroneously operated by such noise. Malfunction of the discriminator is also caused by the absence ofthe second sound carrier wave S2. In such a case, even if the second sound carrier wave S2 is absent, the audio signal may be switched to the sub-channel for stereo or bilingual broadcasting. As a result, a signal having no carrierwave, that is, noise, is supplied to the second sound intermediate frequency amplifier which then produces a frequency demodulated signal at maximum gain,thus producing loud buzz.

According to the present invention there is provided a stereophonic television receiver comprising: a tuner; an intermediate frequency amplifier having an intercarrier detector; first and second sound intercarrier amplifiers having respective frequency modulation detectors; a switching circuit having a matrix circuitfor matrixing the signals from said sound intercarrier amplifiers andforgenerating astereosignal; switching means for selectively supplying output signals of said first and second intercarrier amplifiers or said stereo signal to output lines; a pilot signal discriminatorfordiscriminating a pilot signal included in the second sound carrier and for controlling the state of said switching means according to the modulation of said pilot signal; and a noise detector comprising: a filter circuit for detecting a noise component ofthe output signal from said second sound interfrequency amplifier; a detector for detecting the amplitude of an output from said filter circuit; and a control circuitfor controlling said pilot signal discriminatorto keep said switching means in a predetermined state when said output from said detector is above a predetermined level.

Even if a monaural sound reproduction apparatus is connected to an embodiment of television receiver according to the invention, so that the second sound carrier signal (that is, the sub-channel sound carrier wave) is not present, noise is prevented from causing malfunction ofthe discriminator.

The invention will now be described by way of example with reference to the accompanying draw ings, in which: Figure 1 shows the relative frequencies of signals, for explaining a conventional sound multiplexsystem used in West Germany; Figure 2 is a block diagram of a first embodiment of stereophonic television receiver according to the invention; and Figure3 is a block diagram of a secondembodi- ment of stereophonic television receiver according to the invention.

Referring to Figure 2, in the first embodiment a radio frequency (RF) signal which includes video and audio signals and which is supplied to a tuner 1 having a predetermined high frequency band-width is amplified and frequency-converted to an intermediate frequency (IF) signal. The signal is then supplied to a video IF (VIF) amplifier 2 in which a video signal is extracted by a filter orthe like. The IF signal is then amplified and demodulated. Meanwhile, part of the IF signal is supplied to a quasiparallelton (QPT)3 for amplifying the IF component of the audio signal.

The audio signal component and the centre frequency component ofthe video signal are extracted by a filter or the like and are amplified. Furthermore, the IF audio signal isfrequency-converted bythecentre frequencyofthe IF video signal.The beat signals are extractedfrom the Video and audio signals in accordance with the intercarrier system, thus demodulating afirstsound carrierwave (forthe main channel) of 5.5 MHz and a second sound carrier wave (forthe sub-channel) of 5.742 MHz.These signals are supplied to a first sound IF amplifier4forthe sub-channel. the first sound IF amplifier4 extracts the 5.5 MHzcarrierwave componentofthe main channel audio signal by a corresponding filterorthe like. The extracted component is amplified, amplitude-limited and frequency demodulated. Similarly, the second sound IF amplifier 5 extractsthe 5.742 MHzcarrier component of the sub-channel audio signal. The extracted signal is amplified, amplitude-limited and frequency demodulated.

Thefrequency demodulated signals are then supplied to respective contacts X11 and X12 of a first switch SW1.Thefrequencydemodulatedsignalsare selectively supplied to contacts X21 and X23 of a second switch SW2, and to one input end of a matrix circuit MXthrough the contact X11 of the first switch SW1 The first switch SW1 is controlled and driven by a first control signal from a first detector 65 which forms a pilot signal discriminator 6 in the bilingual broadcast mode. In this case, the switch SW1 is automatically changed overtothecontactX12,thus producing the sub-channel audio signal. Thefirst control signal is supplied to the first switch SW1 through a third switch SW3.The switch SW3 can be opened manually if the user wants to select the main channel signal in the bilingual broadcast mode. The frequency demodulated sub-channel audio signal derived from the second sound IF amplifier 5 is supplied to the other input end of the matrix circuit MX.The matrix circuit MX produces a stereo audio signal which includes a right-channel audio signal R and a left-channel audio signal L in the stereo broadcast mode in accordance with a frequency demodulated main channel audio signal (R+L)/2 and a sub-channel audio signal r. The right and left channel audio signals Rand L are supplied to contacts X22 and X24 ofthe second switch SW2.

The second switch SW2 is controlled and driven by a second control signal from a second detector66 whichformsthepilotsignal discriminator 6 in the stereo broadcast mode. In this case, the second switch SW2 is automatically switched to the contacts X22 and X24,so that the stereo audio signal is selected and produced in the stereo broadcast state..

The first to third switches SW1 to SW3 selectively produce the main channel and sub-channel audio signals and the stereo sound signal. Thefirstto third switches SW1 toSW3andthe matrix circuit MXform a switching circuit SWO.

Meanwhile, partofthe frequency demodulated sub-channel audio signal from the second sound IF amplifier5 is supplied to the pilot signal discriminator 6. The pilot signal discriminator 6 comprises a 3.5fH (where fH is the horizontal scanning frequency) band-pass amplifier (BPA) 61, an AM detector 62, a 274 he filter 63, a 117 Hzfilter 64, and first and second detectors 65 and 66. The pilot signal carrierwave selected and amplified bythe 3.5 fH BPA 61 is detected by the AM detector 62. A DC output is detected in the monaural broadcast, a discrimination signal corresponding to an AC 274 Hz output is detected in the bilingual broadcast, and a 117 Hz discrimination signal is detected in the stereo broadcast. These discrimination signals are supplied to the 274 Hzfilter 63 and the 117 Hz filter 64, respectively.

Thefilters63and 64 discriminatethe types of pilot signals (that is, discrimination signals). The first and second detectors 65 and 66 then detect DC signals, that is, the first and second control signals which are thus supplied to the first and second switches SW1 and SW2. In the case of detecting a 274 Hz discrimination signal (that is, the bilingual broadcast), the first switch SW1 is controlled and operated so asto select the sub-channel audio signal. On the other hand, in the case of detecting a 117 discrimination signal (that is, the stereo broadcast), the second switch SW2 is controlled and operated so as to select the stereo sound signal.

A part of the frequency demodulated sub-channel audio signal from the second sound IF amplifier 5 is also supplied to a high-passfilter (HPF) 7. The high-frequency component excluding the sub-channel audio signal is extracted by the HPF 7. The extracted signal is then supplied to atrap circuit8, thereby eliminating the pilot carrierwave compo- nent. The signal thus obtained is supplied to a control circuit 10through a detector 9.

If the sub-channel audio signal is absent in the signal orthe RF input signal level is extremely weak, the FM output signal amplified bythesound IF amplifier 5 at maximum gain has a strong noise component. This noise component spreads so widely and is so strong that a detected output which has a level higherthan a predetermined level is produced even if the sub-channel audio component and the pilot carrierwave are eliminated. Ifthe detected output from the detector 9 has a level higherthanthe predetermined level, that is, ifthesub-channel sound carrier wave is not present, the pilot signal discriminator 6 is forcibly set in the monaural broadcast mode bythecontrol circuit 10.

For example, as shown in Figure 2, when the collector of a switching transistor of the control circuit loins connected to the input ends of the 274 Hz filter 63 and the 117 Hz filter 64 of the pilot signal discriminator6, and the emitterthereofisgrounded, the switching transistor is controlled such that a detected DCvoltage based on the detection output is supplied to the base of the switching transistor. Therefore, the inputs ofthe filters 63 and 64 are forcibly grounded, thus achieving the monaural discrimination state.

In monaural broadcasting, the pilot signal discriminator6 is controlled so asto select the main channel audio signal by means ofthe first and second switches SW1 and SW2.

Even if the sub-channel sound carrier wave is not present and the presence or absence of the pilot carrierwave (that is, the pilot signal) cannot be detected, buzz is not produced at the monaural sound reproduction apparatus.

It is noted thatthe HPF 7, the trap circuit 8, the detector 9, and the control circuit 10 form a noise detection circuit 11.

Figure 3 is a block diagram showing the overall configuration ofthe second embodiment. The same reference numerals as used in Figure 2 denote the same parts in Figure 3, and a detailed description thereofwill be omitted.

In a frequency-converting operation at a quasiparallelton 3, a beat signal whose frequency is 242 kHz occurs between the main channel carrier (5.5 KHz)and the sub-channel carrier (5.742 MHz). This beat signal is produced buy a cross-talk signal caused by low performance of the filters in the first and second sound lFamplifiers4and 5. Inotherwords, when the sub-channel sound carrier wave is present, a beat signal of 242 occurs as a noise component.

Part ofthe signal from the quasi-parallelton 3 or partofoneofthefrequencydemodulated signals from thefirstand second sound IF amplifiers 4 and 5 is suppliedto a 242 kHz band-passfilter,thus extracting the 242 kHz beat signal. The beat signal is supplied to an amplifier/detector 9' where it is amplified and detected. The detection output from the amplifieridetector 9' is supplied to a control circuit 10' which controls a pilot signal discriminator6 in accordance with a detection output having a level higherthan a predetermined level (that is, in accord ante with the presence or absence of the beat signal).

Otherwise the arrangement of the circuit of Figure 3 is substantiallythe same as that of Figure 2.

In this case a band-pass filter 12, the detector 9' and the control circuit 10' form a noise detection circuit 11'.

Claims (7)

1. Astereophonictelevision receiver comprising: a tuner; an intermeidatefrequency amplifier having an inter carrier detector; first and second sound intercarrier amplifiers having respective frequency modulation detectors; a switching circuit having a matrix circuit for matrixing the signals from said sound intercarrier amplifiers and for generating a stereo signal; switching means for selectively supplying output signals of said first and second intercarrier amplifiers or said stereo signal to output lines; a pilot signal discriminatorfor discriminating a pilot signal included in the second sound carrier and for controlling the state of said switching means according to the modulation of said pilot signal; and a noise detector comprising: a filter circuit for detecting a noise component ofthe outputsignalfrom said second sound interfrequency amplifier; a detectorfordetecting the amplitude of an output rrom said filter circuit; and a control circuitfor controlling said pilotsignal discriminatorto keep said switching means in a predetermined state when said output from said detector is above a predetermined level.

2. A stereophonic television receiver according to claim 1 wherein said predetermined state of said switching means is a monaural state.

3. Astereophonictelevision receiver according to claim 1 wherein said predetermined state of said switching means is selected for supplying the output signal from said first interfrequency amplifierto said output lines.

4. A stereophonictelevision receiver according to claim 1 wherein said filter circuit ofthe noise detector includes a trap for rejecting a carrier signal of said pilot signal.

5. Astereophonictelevision receiver according to claim 1 wherein said filter circuit of the noise detector includes a band-pass filterfor passing a beat signal generated between first and second sound carrier signals.

6. A stereophonic television receiver substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

7. A stereophonic television receiver substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.